1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor in the current-perpendicular-to-plane (CPP) configuration, more specifically to the stabilization of the magnetization of the free layer of such a sensor.
2. Description of the Related Art
The synthetic (Sy) spin-valve (SV) giant magnetoresistive (GMR) sensor is a multi-layered configuration of magnetic and non-magnetic layers which includes a magnetically free layer, whose magnetic moment is free to respond to outside magnetic stimuli, separated by a non-magnetic layer from a magnetically “pinned” layer, whose magnetic moment is fixed in direction. The motion of the free layer magnetic moment relative to the pinned layer magnetic moment changes the resistance of the sensor so that a “sense” current passing through the layers produces measurable voltage variations across the sensor. The adjective “synthetic,” applied to the sensor refers to the structure of the pinned layer which is formed as two layers of ferromagnetic material, magnetized in opposite directions, and held in that antiparallel configuration by a non-magnetic layer of material placed between them. The separation of the two layers by the proper distance (and proper material) produces an exchange coupling between their magnetic moments which is energetically preferentially anti-parallel, so the resulting tri-layer is called a “synthetic antiferromagnetic” layer. The synthetic tri-layer is typically formed on a “pinning” layer of legitimately antiferromagnetic material, which serves to hold the entire structure in a stable configuration of its magnetic moments.
The synthetic spin valve is presently fabricated in two basic configurations, current perpendicular to plane (CPP) and current in plane (CIP), which differ basically in the path taken by the sense current as it passes through the sensor stack (ie. its configuration of layers). As can be inferred from the terminology, the sense current in the CPP configuration passes vertically, from top to bottom, perpendicularly to the planes of the stack layers. In the CIP configuration, the current passes horizontally, from side to side, within the planes of the stack layers.
One of the problems associated with the design of a good spin valve sensor is the stabilization of its magnetically free layer. In general, the magnetization of the free layer would be fragmented into a multiplicity of domains. This domain structure is not fixed in time, however, but moves randomly as a result of thermal agitation and/or external fields, leading to a form of noise in the sensor signal called Barkhausen noise. It is, therefore, desirable to maintain the free layer in a single domain state, which would be much less subject to agitation and this process is called stabilization. Since the magnetization of the free layer is typically directed “longitudinally” (within the plane of the layer and within the plane of the air-bearing-surface (ABS) of the sensor), whereas the magnetization of the pinned layer is directed transversely to the ABS plane, the stabilization of the free layer is accomplished by what is called a “longitudinal bias layer (LBL) (or biasing layer).” In the CIP configuration, the longitudinal biasing layer is formed as two layers, one on either side of the stack configuration. These layers are generally permanent magnets (called “hard bias”), which couple magnetostatically to the lateral edges of the free layer and produce the single domain. Since the sense current must go through the plane layers from one side to the other, the biasing layers, which are typically conductors, do not impede the operation of the sensor. It must be pointed out, however, that as the width of the sensor (its “trackwidth”) becomes narrower, the longitudinal biasing effect of the laterally positioned magnets can actually impede the free movement of the free layer magnetic moment as it responds to external magnetic fields. For this reason, among others, alternative forms of longitudinal biasing have been developed.
In the CPP configuration, however, placing conducting permanent magnets on each side of the layer configuration produces an even more severe and immediate problem, since the magnets would give the sense current a pathway that bypasses the sensor configuration. Although this problem can be ameliorated by placing insulating layers between the biasing layers and the sensor, this solution increases the difficulties of fabricating the sensor. Therefore, an alternative biasing configuration, called “in-stack stabilization,” has been used, in which a biasing layer is formed over the free layer, rather than to either side of it. Since all layers have a common horizontal cross-section, this allows the entire configuration to be formed in a self-aligned scheme, making the fabrication process relatively simple. The in-stack approach is discussed in great detail in Smith et al. (U.S. Pat. No. 6,473,279 B2). Smith et al. teach the formation of an a first auxiliary ferromagnetic layer above the free layer which couples antiferromagnetically to the free layer by means of exchange coupling (RKKY coupling) across a non-magnetic coupling layer and a second auxiliary exchange pinning layer, which exchange pins the first auxiliary layer.
Mao (U.S. Pat. No. 6,466,419 B1) teaches a CPP spin valve structure wherein a spacer layer is formed on the free layer and a biasing layer of antiferromagnetic material is formed on the spacer layer. In addition, to prevent unwanted side readings by the free layer, the entire configuaration is laterally covered by an insulation layer and then covered by a concave shield.
Nishiyama (U.S. Patent Application Publication No.: US 2003/0053269 A1) teaches a method of forming a CPP in which the lateral sides of the CPP stack are sloped to the vertical and have two different slope angles, this configuration being claimed to prevent shorting between the stack layers and the upper and lower electrodes. In addition, the ferromagnetic free layer of the sensor is exchange coupled in an antiferromagnetic configuration with an additional ferromagnetic layer formed above it and separated from it by a non-magnetic layer.
Hasegawa (U.S. Patent Application Publication No.: US 2003/0143431 A1) discloses a CPP configuration of two stacked dual spin valve sensors, each of the dual spin valve sensors including a free layer positioned between an upper and lower synthetic pinned layer. A hard magnetic layer sandwiched between the two dual spin valves serves as an in-stack biasing layer.
An alternative in-stack configuration, called a “stitched” in-stack configuration, has been developed for the purpose of broadening the current profile in the antiferromagnetic layer which is used to pin the longitudinal biasing layer. Referring to FIG. 1a, there is seen a basic (non-stitched) bottom spin valve CPP stack having a uniform horizontal cross-section of its layers, a so-called “pillar” stack formation. Moving vertically from the bottom up, there is seen a seed layer (10), a first antiferromagnetic (AFM) pinning layer (20), a synthetic antiferromagnetic (SyAF) pinned layer (30) which is a tri-layer formed of a first ferromagnetic layer (32), denoted “AP2 (for antiparallel magnetization), a non-magnetic coupling layer (34) and a second ferromagnetic layer (36), denoted AP1. The magnetizations of (32) and (34) are antiparallel and out of the plane of the figure (the ABS plane) as shown by the arrowhead and arrowtail symbols. A non-magnetic spacer layer (40) separates the SyAF from the ferromagnetic free layer (50). The ferromagnetic free layer is formed on the spacer layer. A non-magnetic coupling layer (60) (more correctly termed a decoupling layer) is formed on the ferromagnetic free layer and a ferromagnetic longitudinal biasing layer (65) is formed on the coupling layer. The role of the decoupling layer (60) is to separate the biasing and free layers sufficiently to avoid exchange coupling and also to mix the spins of the electrons so that the biasing layer (65) plays no role in determining the GMR properties of the stack. The magnetization directions of the free layer and the biasing layer are set longitudinally in opposite directions and the magnetization of the biasing layer is pinned by an antiferromagnetic (AFM) layer (70) formed on the biasing layer. It is shown by simulation analysis that the magnetic thickness (product of the magnetic moment and actual thickness) of the biasing layer must be approximately 1.5 to 2 times that of the free layer for sufficient stabilization to occur.
Referring to FIG. 1b, there is shown the (prior art) stitched configuration of the CPP sensor. This form is identical to the non-stitched version except that after the foregoing layers (10)–(65) are patterned to a uniform horizontal cross-section, ie the pillar portion of the sensor, a second portion of a longitudinal bias layer (71) of greater horizontal cross sectional area is then “stitched” onto the first portion (65) and an antiferromagnetic pinning layer (80), of substantially equal area, is formed on the stitched portion of the bias layer. The stitched bias layer is coupled to the free layer and is pinned by the AFM pinning layer. The extended cross-sectional area of the AFM layer (80) allows a broader horizontal current distribution (more uniform current profile) and a more uniform current through the remainder of the stack, thereby improving the sensor performance by allowing higher sense current through the sensor and as measured by the GMR ratio. In addition, the stitched configuration also improves the exchange coupling of the longitudinal bias layer. In the pillar design, the coupling between the bias layer and the free layer is ferrimagnetic to ferromagnetic coupling, whereas in the stitched design, the coupling within the neck area is ferromagnetic to ferromagnetic (since it is two ferromagnetic layers (71) and (65) that are in contact).
Although the stitched configuration improves the performance of the sensor because of the improved current profile, the stabilization of the free layer is relatively poor, as can be seen by examining transfer curves (rotation of the free layer magnetic moment as a function of external magnetic field). Referring to FIG. 2 there is seen the simulated transfer curves of a stitched configuration for two currently achievable exchange fields between the free and bias layers obtainable for a free layer of 30 angstroms thick CoFe and a bias layer of between 45–60 angstroms of CoFe equivalent. The solid line curve represents an exchange field of 650 Oe, corresponding to a bias layer of magnetic thickness 1.5 times the magnetic thickness of the free layer. The broken line curve corresponds to an exchange field of 500 Oe representing a bias layer whose magnetic thickness is 2 times the magnetic thickness of the free layer.
In order to suppress the degree of hysteresis exhibited by the transfer curve it is found that an exchange field of at least 800 Oe is required for the 1.5× case and at least 1100 Oe is needed for the 2× case. It is the objective of this invention to provide a method for producing just such an increased exchange field within the stitched in-stack biasing configuration.